Low temperature reduction process for large photomasks

ABSTRACT

Glass photomasks having a high resolution stain pattern for use in photolithographic processes are made by migrating stain-producing cations into a surface of the glass, and heating the glass containing the stain-producing cations in a pressure vessel containing an inert liquid saturated with hydrogen under pressure at relatively low temperatures to reduce and agglomerate the stain-producing cations to form a high resolution stain pattern.

FIELD OF THE INVENTION

The present invention relates generally to the art of producing a stainpattern in a glass substrate, and more particularly to the conditionsfor reducing and agglomerating the stain-producing cations in largeglass substrates.

BACKGROUND OF THE INVENTION

Photomasks are used in photolithographic processes for printingelectronic circuits and other precision photofabricated parts. In atypical photolithographic process, a substrate is covered with a layerof photoresist material over which a photomask is superimposed. Thephotomask has a pattern of opaque and transparent areas with respect toactinic radiation, typically ultraviolet light, which is passed throughthe photomask to reproduce the pattern in the photoresist material. Thepattern is developed as a relief image in the photoresist material bymeans of different solubilities of the exposed and unexposed portions ofthe photoresist material.

Since the preparation of a photomask involves a substantial amount oftime, labor and material, it is desirable that a photomask besufficiently durable for repeated use in the manufacture ofphotofabricated articles. It is also desirable to maximize theresolution of the pattern carried by a photomask in order to improve theprecision of the image it transfers to the photofabricated articles.

Photolithographic processes have employed photomasks comprising a sheetof glass carrying a patterned coating of chromium, iron oxide orphotographic emulsion. While iron oxide and chromium films areconsiderably more durable than photographic emulsions, all coatedphotomasks are subject to scratching and other mechanical damage whichshortens their useful life. In addition, the etching required to producea desired pattern in chromium or iron oxide films produces a loss ofresolution as a result of the etch factor, the fact that an etchedgroove grows wider as it grows deeper.

Photomasks of improved durability comprising a stained pattern within aglass substrate are disclosed in U.S. Pat. No. 3,573,948 to Tarnopol andU.S. Pat. No. 3,732,792 to Tarnopol et al. Although these stained glassphotomasks have improved durability, the step of etching a patternthrough a stained layer of the glass in the former or the step ofetching through a tin oxide coating in the latter results ininsufficient resolution for some applications. U.S. Pat. No. 3,561,963to Kiba discloses a stained glass photomask wherein the desired patternis etched into a copper film on a glass substrate, and copper ions aresubsequently migrated into the glass by heating. Although the stainedphotomask pattern is more durable than a coating, resolution iscompromised in this process as a result of the etching of the film andthe migration step which results in lateral spreading of the stainedareas into the adjacent unstained areas.

U.S. Pat. No. 2,927,042 to Hall et al. and U.S. Pat. No. 3,620,795 toKiba disclose methods designed to minimize the lateral diffusion ofstaining ions in the aforementioned processes. The Hall patent describesdepositing a film of stain-producing metal onto glass and removingportions of the film by photoetching. An electrical field is then passedthrough the glass so that the patterned film migrates into the glasssubstrate. The Kiba patent discloses etching a pattern into a metal filmand migrating stain-producing ions through apertures in the metal filmby heating in an electric field. Both methods suffer a loss ofresolution as a result of the etching step. U.S. Pat. Nos. 2,732,298 and2,911,749 to Stookey both disclose the production of a stained imagewithin a glass plate by heating a developed silver-containingphotographic emulsion on the glass. However, the use of relatively hightemperatures of 400° to 650° C. results in a loss of resolution of thestained pattern.

U.S. Pat. No. 4,155,735 to Ernsberger discloses an improved method formaking stained glass photomasks. The method comprises developing apatterned photoresist layer on a glass substrate and applying anelectric field to enhance the migration of staining ions throughapertures in the photoresist pattern into the surface of the glasssubstrate. The staining ions are then reduced and agglomerated to form astained pattern within the surface of the glass by heating the glass inthe presence of a reducing agent such as tin or copper ions or in areducing atmosphere such as forming gas, preferably at temperatures ofabout 400° to 500° C.

In U.S. Pat. No. 4,309,495, Ernsberger describes producing stained glassphotomask patterns by exposing and developing a photographic emulsion ona sheet of glass and migrating silver ions from the emulsion into thesurface of the glass under the influence of an electric field andmoderately elevated temperatures. These silver ions are then reduced andagglomerated to form a stained pattern within the surface of the glassby maintaining the glass at an elevated temperature in the presence of areducing agent. The reducing agent may be reducing ions, such as cuprousions migrated into the glass, or the stannous ions inherently presentnear the surface of glass produced by the float process, in which casean optimized rate may be obtained at temperatures of 475° to 525° C. Inan alternative embodiment, the reducing agent may be a reducingatmosphere such as forming gas in the heating chamber during thereducing and agglomerating heat treatment, in which case practical ratescan be realized at lower temperatures in the range of 350° to 400° C.

Microscopic examination of stained patterns in glass photomasks made inaccordance with the aforementioned methods shows that the pattern edgesare slightly blurred. A microdensitometer scan shows that the opticaldensity profile of such an edge is sloped, requiring as much as fifteenmicrons (about 0.006 inches) to go from maximum to minimum density. Thissloping profile is known as the "roll-off region". The ideal opticaldensity profile of an edge, which could be described as perfect edgedefinition, would be rectangular. For certain purposes, such asphotomasks intended for use in the silicon integrated circuit industry,pattern lines only five to ten microns wide are necessary. Therefore,edge definition must be very good. A roll-off region of one micron widthmay be the upper tolerable limit.

In U.S. application Ser. No. 323,333 filed by the same inventor on evendate herewith, a method for producing high resolution stained glassphotomask patterns is disclosed. The method includes electromigration ofstain-producing ions as described above. However, instead of reductionand agglomeration at temperatures above 400° C. in the presence ofreducing ions or at temperatures of 350° to 400° C. in a forming gasatmosphere as taught in the prior art, the stained pattern is producedby reduction and agglomeration of the stain-producing ions in thepresence of pure hydrogen at superatmospheric pressure at temperaturesbelow 300° C., preferably below 200° C., and at pressures preferablybetween ten and 100 atmospheres. Such pressures are readily and safelyattainable only for relatively small substrates.

SUMMARY OF THE INVENTION

The present invention relates to the production of high resolution stainpatterns within a large glass substrate. According to the presentinvention, stain-producing ions are electromigrated into the glass atrelatively low temperatures (around 200° C.) as in previous methods.However, rather than reducing and agglomerating the stain-producing ionsat relatively high temperatures in the presence of reducing agents suchas cuprous or stannous ions or in a forming gas atmosphere as inprevious methods, the present invention involves the reduction andagglomeration of the stain-producing ions in a pressure vesselcontaining an inert liquid saturated with hydrogen under pressure.Temperatures in the same range as the temperatures utilized during theelectromigration of the stain-producing ions, typically 150° to 200° C.,are sufficient to produce high resolution stain patterns in the glassusing pure hydrogen in an inert liquid as the reducing agent atpressures above atmospheric, preferably pressures up to about tenatmospheres.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The cited references illustrate the development of methods for producingdurable stained patterns in glass photomasks. However, certainphotolithographic processes require high resolution patterns withpattern lines only five to ten microns in width and with very sharp edgedefinition.

It has been discovered that, while the pattern of injected staining ionsis initially very sharp, edge definition deteriorates during the hightemperature reduction and agglomeration of the staining ions to producethe photomask pattern. It has been discovered that the principal reasonfor this deterioration of the edge definition of the pattern is randomdiffusion of the stain-producing ions during the early stages of thereduction treatment. The stain-producing ions that are not yet reducedand incorporated into a pattern of colloidal particles diffuse in alldirections including laterally, that is, in a plane parallel to thesurface. Thus, a portion of the stain-producing ions diffuse outside theintended limits of the pattern before encountering a reducing agent andbeing converted to relatively immobile stain particles.

Another factor that can adversely affect edge definition is theinjection of too great a quantity of stain-producing ions during theelectromigration step. The intense electrical field within the glasstends to suppress lateral motion of the stain-producing ions as they areinjected into the glass. However, the electrical field is not trulyunidirectional at any time after the injection of stain-producing ionsbegins, because the region of glass in which sodium ions have beenreplaced by stain-producing ions has substantially higher resistancethan the remainder, leading to the development of a divergent fringefield at the edges of the pattern. As a result of this fringe field, theprofile of stain-producing ion injection is presumably trapezoidal. Thecharacteristic angle of this trapezoid is not known, but could be aslarge as 135 degrees, in which case the edge of the pattern would bedisplaced by one micron for each micron of injection depth. Therefore,edge definition may be improved by limiting the depth of injection ofstain-producing ions, preferably to one micron or less.

The diffusive spreading of the stain-producing ions can be diminished bycarrying out the reduction and agglomeration steps at lowertemperatures. However, the rate of reduction would likewise bedecreased. The compensating increase in the time required for reductionwould give more time for diffusive spreading, thereby nullifying thebenefit of the lower temperature. This logical consequence is, in fact,found to exist in practice when the stain-producing ions are reduced byreducing ions such as tin or copper in the glass surface layer. However,it has been discovered in accordance with the present invention thatwhen pure hydrogen is the reducing agent, there is a substantial netbenefit in lowering the temperature of the operation, especially atincreased pressure.

The explanation for this benefit from using hydrogen under pressure asthe reducing agent at lower temperatures may be explained by the natureof the diffusion of hydrogen in glass. The activation energy forpermeation of hydrogen in vitreous silica is known to be about seven toeight kilocalories per gram atom, while the activation energy for silverions, for example, is at least 35 kilocalories per gram atom. Aprinciple of physical chemistry is that the temperature coefficient ofan activated process is related in an exponential manner to theactivation energy of that process. Accordingly, the rate of silverdiffusion drops steeply with decreasing temperature, becoming almostnegligible at 150° C., while the diffusion of hydrogen declinesrelatively little.

Despite the low activation energy, transport of hydrogen in glass isinherently slow because of the low solubility of hydrogen in glass. Theconcentration of dissolved hydrogen can be increased to useful levelsfor the reduction of stain-producing ions to form photomask patterns inaccordance with the present invention by employing hydrogen underpressure. Moreover, the utilization of stain-producing ions inaccordance with the present invention is so efficient that the depth ofinjection of stain-producing ions may be limited to one micron or less.

There is substantial latitude in the choice of process variables thatwill produce high resolution photomask patterns in glass substrates inaccordance with the present invention. Preferred conditions includetemperatures below 300° C., preferably below 200° C., and pressuressubstantially above atmospheric. While pressures of ten to 100atmospheres may be readily and safely attainable for relatively smallsubstrates, the problems of building a large autoclave capable ofwithstanding such pressures, as well as the safe handling of largequantities of hydrogen under such pressures, are significant. Thepresent invention avoids these problems, while still obtaining efficientreduction and agglomeration of stain-producing ions using hydrogen underpressure as the reducing agent. According to the method of the presentinvention, large substrates are placed in a large pressure vessel whichis filled with an inert liquid saturated with hydrogen under pressure.The reduction and agglomeration of stain-producing ions may beefficiently carried out at moderate temperatures, below 300° C. andpreferably below 200° C., at pressures up to about ten atmospheres.

Glass compositions useful for photomask substrates in accordance withthe present invention are those containing mobile cations capable ofbeing electromigrated at moderate voltages to provide sites into whichthe stain-producing cations may be injected. Alkali metal ions such assodium, potassium and lithium have relatively high mobility in glass.Thus, glasses having at least minor amounts of alkali metal oxides areparticularly useful. For example, conventional soda-lime-silica glasscompositions typically contain about ten to thirteen percent by weightsodium oxide and a trace of potassium oxide, which represents a morethan adequate supply of mobile cations. Other glass compositions havinglower alkali metal oxide concentrations may also be used with thepresent invention, limited only by the ability to develop a stainpattern with sufficient density to mask the actinic radiation to be usedin a subsequent photofabrication process.

Photoresist materials useful in accordance with the present inventionare defined as polymeric materials which upon exposure to actinicradiation, typically ultraviolet light, develop areas which are solublein a particular solvent and other areas which are insoluble. Whenexposed to the particular solvent, the soluble areas are removed,leaving a pattern of apertures in the photoresist layer through whichstain-producing ions may be migrated. Specific examples of commerciallyavailable photoresists which may be used in accordance with the presentinvention include LSI-195 Photoresist sold by Philip A. Hunt Company,KPR and KFTR Photoresist sold by Eastman Kodak Company, and AZ-111 andAZ-1305J sold by the Shipley Company.

The layer of stain-producing ions may be applied as a compound of one ormore of the stain-producing cations having relatively low electricalconductivity or as a metallic film by employing conventional coatingtechniques such as evaporation, sputtering, wet chemical deposition andother known techniques. Preferred stain-producing cations includesilver, copper, gold, thallium and mixtures thereof.

Preferably, migration of the stain-producing ions into the glass surfaceis carried out by applying electrically conductive layers over bothsides of the substrate and applying an electrical potentialtherethrough. The electrically conductive layers preferably comprisecolloidal graphite, which may be applied to the substrate in aqueous oralcoholic slurry form or as an aerosol spray.

Application of an electric field between the electrode layers drivesmobile cations deeper into the glass substrate while causing thestain-producing ions to be injected into the glass into the spacesvacated by the displaced alkali metal cations. The rate of ion migrationis determined by the applied voltage and the temperature. At ambienttemperatures the rate of ion migration is relatively slow; therefore,elevated temperatures above about 100° C., preferably from about 100° to200° C., are preferred in order to obtain reasonable ion migration timesat an electric potential of a few hundred volts.

After the stain-producing ions have been electromigrated into the glassto the desired depth, preferably one micron or less, development ofoptical density in the ion-migrated zones is obtained by heating theglass substrate in the presence of a reducing agent to reduce thestain-producing ions to their elemental state and then to agglomeratethe metallic atoms into a submicroscopic crystalline form. These stepsare carried out in accordance with the present invention in the presenceof an inert liquid saturated with hydrogen at relatively lowtemperatures, preferably less than 300° C. and more preferably less than200° C., at pressure above atmospheric, preferably on the order of twoto ten atmospheres.

Numerous configurations of stain-producing ions and photoresistmaterials are useful in accordance with the present invention. Forexample, a photoresist material may be applied directly onto the glasssubstrate, a pattern developed in the photoresist, then a layer ofstain-producing ions applied over the photoresist material.Alternatively, a layer of stain-producing ions may be deposited onto theglass surface and a photoresist material pattern developed thereover.Various configurations are discussed in detail in U.S. Pat. No.4,155,735, the disclosure of which is incorporated herein by reference.

In a preferred embodiment of the present invention, a high resolutionstained glass photomask is made by developing a silver-containingphotographic emulsion on a glass substrate and migrating silver from thedeveloped photographic emulsion into the glass. Silver ions migrate intothe glass, displacing mobile cations which migrate deeper into the glasssubstrate. The injected silver ions are then reduced to the elementalstate and agglomerated into submicroscopic crystals within the glass toproduce a stain pattern by heating in the presence of an inert liquidsaturated with hydrogen under pressure in accordance with the presentinvention. Photographic emulsions useful in producing stained glassphotomask patterns in this embodiment of the invention are those capableof being developed to produce a residual layer of emulsion and silver orsilver halide, which has sufficient electrical conductivity to permitelectromigration of silver ions from the emulsion into the glasssubstrate. The emulsion should also have high resolution capability inorder to produce a high resolution photomask pattern.

Standard photographic techniques are employed to expose and develop thephotographic emulsion. The glass substrate bearing a film ofphotographic emulsion is exposed to actinic radiation through a masterpattern in order to form a latent image, which is subsequently developedin the photographic emulsion by exposure to appropriate developingsolutions. Either a positive image or a negative image may be developedon the substrate depending on the type of photographic emulsion employedin the developing process. The electric field employed to migrate thesilver ions from the developed photographic emulsion into the subjacentglass surface is preferably high enough to migrate the necessaryquantity of silver ions within a reasonable time, but low enough toavoid arcing around the edges of the glass substrate between the anodeand cathode layers. Typically voltages of 50 to 1000 volts, preferably200 to 700 volts, are sufficient at temperatures of about 100° to 300°C. In order to maximize the resolution of the pattern in accordance withthe present invention, the depth of silver ion migration is preferablylimited to one micron or less, which is sufficient in view of theefficiency of reduction and agglomeration of the stain-producing ions inthe presence of an inert liquid saturated with hydrogen under pressurein accordance with the present invention. The inert liquid, i.e.chemically inert to molecular hydrogen under the temperature andpressure conditions of the present invention, is preferably ahydrocarbon or fluorocarbon having a vapor pressure less than oneatmosphere at the reduction and agglomeration temperatures of thepresent invention.

Once a sufficient quantity of ionic silver has been electromigrated intothe glass substrate to the desired depth, the stain pattern is developedby reduction of the silver ions to the elemental state and agglomerationinto submicroscopic crystals by heating in the presence of an inertliquid saturated with hydrogen under pressure. While temperatures aboveabout 400° C. are generally required for the reduction and agglomerationsteps to proceed in an ambient atmosphere, and temperatures of about350° to 400° C. are required for the reduction and agglomeration stepsto proceed in a forming gas atmosphere, the presence of an inert liquidsaturated with hydrogen under pressure as the reducing agent inaccordance with the present invention allows the reduction andagglomeration of silver to proceed at temperatures below 300° C. andpreferably below 200° C. in order to maximize the resolution of thestained photomask pattern. At the temperatures and pressures inaccordance with the present invention, a density of at least 2.0 withrespect to ultraviolet radiation may be obtained in about four tosixteen hours.

The present invention will be further understood from the descriptionsof specific examples which follow.

EXAMPLE I

A test sample cut from a production photomask 22×28×0.190 inches (about56×71×0.5 centimeters) after electromigration treatment but beforereduction, is placed in a four ounce wide-mouth glass bottle and GulfDXE® fluid (a commercial heat transfer fluid, di-orthoxylyl ethane) isadded until half the length of the sample is immersed in the liquid. Theopen bottle containing the sample and the liquid is placed in a oneliter pressure vessel (Parr Instrument Company, No. 4611). Air is purgedfrom the vessel by filling twice to 500 pounds per square inch withforming gas and twice releasing the pressure to ambient; then the vesselis pressurized to 360 pounds per square inch with pure hydrogen andsealed. The vessel is then heated to 180° C. for 14 hours. The pressureinside the vessel increases to 460 pounds per square inch as aconsequence of the heating. Measurements on the sample after theconclusion of this treatment show an ultraviolet optical density of 3.21in the portion of the surface that was submerged in oil, and 3.05 in theportion contacted only by gaseous hydrogen. Optical density in thevisual region of the spectrum (defined by Wratten 106 filter) is 0.95 inthe submerged portion and 0.90 in the remainder. The color is a deepreddish amber. Microscopic examination of the stain pattern at 200X,with a resolution of about one micron, reveals no observable roll-offregion. Properties of the fluid are unchanged. An identical sample,processed in forming gas for three hours at 343° C., has an ultravioletoptical density of only 2.30, and an olive-green visual appearance. Anexamination of complete absorption spectra for both stains shows thatboth have an absorption maximum in the region around 400 nanometers, butthe absorption band for the stain produced at higher temperature isbroader, consistent with the fact that a wide variety of silver particlesizes is present.

EXAMPLE II

Glass substrates containing a latent image formed by electromigration ofsilver ions are placed in an autoclave welded from ordinary mild steel.Most of the remaining space in the autoclave is filled with paraffinoil. Hydrogen is added to produce a pressure of about ten atmospheres at180° C. After twelve hours the glass photomask bears a stained imagewith a density above 2.0 with respect to ultraviolet radiation. Thisdensity is comparable to that obtained after three hours of reductionand agglomeration in forming gas at 400° C., at which temperature edgedefinition is compromised by lateral diffusion of the silver stainingions.

The above example is offered to illustrate the present invention.Various other inert liquids such as 1,1-di(orthoxylyl)ethane, mineraloil and fluorocarbons may be employed. Other stain-producing ions suchas gold, copper and thallium, and a wide range of treatment times,temperatures and pressures may be employed limited only by the strengthof the pressure vessel and the required resolution of the stain pattern.The scope of the present invention is defined by the following claims.

I claim:
 1. In a method for producing a stain pattern in a glasssubstrate comprising the steps of injecting stain-producing cations intoa surface of a glass substrate and heating the glass in the presence ofa reducing agent to reduce and agglomerate the stain-producing cationsto produce a stained pattern within the surface of the glass, theimprovement which comprises heating the glass in a pressure vesselcontaining an inert liquid saturated with hydrogen under pressure. 2.The method according to claim 1, wherein the stain-producing cations areselected from the group consisting of silver, copper, gold and thallium.3. The method according to claim 1, wherein the step of injectingstain-producing ions into the glass surface is carried out by applyingan electric field at a temperature from about 100° to about 200° C. 4.The method according to claim 1, wherein the stain-producing ions areinjected into the glass surface to a depth not greater than one micron.5. The method according to claim 1, wherein the step of reducing andagglomerating the stain-producing ions is carried out at a temperaturebelow 300° C. and a pressure of about two to ten atmospheres.
 6. Themethod according to claim 1, wherein the inert liquid is selected fromthe group consisting of hydrocarbons and fluorocarbons having a vaporpressure of less than one atmosphere at temperatures up to about 300° C.7. An article of manufacture prepared according to the method of claim1, wherein the roll-off region of the stained pattern edges is not morethan one micron.
 8. An article of manufacture prepared according to themethod of claim 4, wherein the roll-off region of the stained patternedges is not more than one micron.
 9. An article according to claim 8,wherein the stain-producing ion is silver.
 10. An article according toclaim 9, wherein the stain pattern has an optical density of at least2.0 with respect to ultraviolet radiation.